Add callback support

Add callback support

This patchs provides the code to add callbacks per event or for each
events. For now the priority is not handled so the callbacks are
processed in a FIFO manner and global callbacks are processed before
per-event callbacks.

Signed-off-by: Julien Desfossez <julien.desfossez@polymtl.ca>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>